init.sh: run tr in the "C" locale to avoid multibyte interpretation

init.sh: run tr in the "C" locale to avoid multibyte interpretation

* tests/init.sh (rand_bytes_): Run tr in the "C" locale so it does
not try to interpret its random input bytes.  Jarno Rajahalme reported
that ./test-xalloc-die.sh would fail with "tr: Illegal byte sequence"
on Darwin 10.3.0 with LC_CTYPE=UTF-8.
(mktempd_): Likewise, just in case.
(cherry picked from commit 6b8b58b99d6aaedeed12300cc154af7d3af7b95f)